Internal lesions caused by infectious pathogens, bruise damage, and bulb malformations are some features that can be detected by the methods proposed. Such an evaluation system could be used to ensure that only high-quality, healthy onions that have a high probability of storing for long periods enter controlled atmosphere storage. Additional quality factors such as bulb shape, consistency, and internal structure can be evaluated, leading to a premium onion grade that would command higher market prices, further increasing the value proposition of such a system to onion producers.
Worldwide, the onion (Allium cepa) is one of the most widely cultivated vegetable crops, second only to the tomato. Worldwide production of onions was 78.5 million metric tons in 2010. That same year, total harvested land area devoted to onion cultivation exceeded four million square hectares. In the United States, 2010 onion production was an estimated 3.72 million metric tons, with an aggregate farm gate value of approximately $1.15 billion.
The sweet yellow Granex varieties of onion grown in Georgia are collectively called Vidalia sweet onions and are a valuable economic commodity and have proven to be highly desirable to consumers due to their sweet and mild flavors. Vidalia onions are only harvested during a brief period of time every year, and the sweet flavor and high water content that makes them desirable to consumers also confer susceptibility to disease and decay, making it difficult to store them for long periods of time after harvest. Onion growers have attempted to extend the market availability of Vidalia onions by storing them in controlled atmosphere facilities, but disease outbreaks still frequently occur in these storage facilities, leading to substantial economic losses for the onion producers. Currently, onions entering storage are screened by human visual inspection for external signs of disease or damage that can increase disease susceptibility, but many onion diseases and defects manifest inside the bulb and can pass human inspection undetected.
Onions are biennial plants, growing vegetatively during the first year of life to maximize sugar production for storage in the bulb. As the plant matures, the leaves die and the plant enters a dormant stage until the next growing season when the plant produces flower stalks and seeds. These growth stages are regulated by the day-night cycle, and Granex varieties belong to a class of onion cultivars known as short-day (SD) onions due to the relatively short photoperiod (11-12 h) required to induce bulb formation in the plant. In the southern latitudes of the US, greenhouse-grown plantlets are typically transplanted to production fields in the late fall, where they grow vegetatively during the short winter days. As day length increases, plant growth shifts from leaf formation to bulb formation, and the onion bulbs are harvested from late April to early May when the bulbs reach a marketable size.
As harvest time approaches, the onion plant's leaves begin to break down, signaling that vegetative growth has stopped and that the plant is entering dormancy. When the onion leaves fall the bulbs can be harvested. A mechanical blade is run underneath the onion bulbs to sever the roots, and the bulbs are lifted out of the ground by the leaves and are left to cure for several days. Excessively hot or cold temperatures as well as heavy rains during the harvest and field curing of the onions can lead to increased incidence of bacterial and fungal rots that adversely affect their storability, leading to substantial losses for the growers. After field curing, the dead leaves are cut off, and bulbs are collected and transported to a sorting facility. There, human inspectors check for signs of disease or damage to the bulbs, and the bulbs are sorted by size. While human visual inspection can be useful in culling bulbs with obvious external defects, many bulbs may have internal damage or latent infections that are not detectable by eye.
While Vidalia onions remain popular with consumers because of their sweetness and lack of pungency, there are associated drawbacks. Laboratory tests of extracted onion flavor compounds have been shown to inhibit the growth of pathogenic fungi and bacteria. Vidalia onions are thought to be particularly susceptible to fungal and bacterial diseases because of their lack of these protective compounds. This increased susceptibility to disease has been a particular problem with regard to controlled atmosphere storage of onions for later sale. During particularly severe disease outbreaks, growers can lose more than 50% of their stored onions to postharvest diseases.
There are numerous pathogens and damage modes that can lead to diminished quality or complete loss of harvested onions, particularly as they are stored for increasing durations in controlled atmosphere facilities. Three of the most common post-harvest diseases affecting sweet onions are Botrytis fungal neck rot, Burkholderia cepacia bacterial rot, and Pseudomonas viridiflava bacterial rot.